1. Field of the Invention
The present invention relates to a photosensor system including a photosensor array constituted by two-dimensionally arranging a plurality of photosensors, and a driving control method of the photosensor in the photosensor system.
2. Description of the Related Art
In recent years, a solid image pickup device has frequently been used, including a photosensor array constituted by arranging a plurality of photoelectric conversion devices (photosensors) in a linear or matrix form in order to pick up a subject image and convert the image to an image signal in image pickup apparatuses for picking up the subject image such as an electronic still camera and video camera, and an image read apparatus for reading a printed matter, photograph, and fine concave/convex shape of a fingerprint.
As this solid image pickup device, in general, a charge-coupled device (CCD) has been used. As well known, the CCD includes a constitution in which a plurality of photosensors such as photodiodes and thin film transistors (TFT) are arranged, and detects a charge amount of electron-positive hole pairs generated in accordance with a light amount with which a light receiving portion of each photosensor is irradiated by horizontal and vertical scanning circuits to sense luminance of an irradiation light.
In the photosensor system using this CCD, selection transistors need to be individually disposed in order to bring each scanned photosensor into a selection state. Therefore, there is a problem that with an increase of the number of pixels constituting the photosensor array, the system is accordingly enlarged.
On the other hand, as a constitution for solving the problem, we have developed a photosensor (hereinafter a double gate type photosensor) by a thin film transistor having a so-called double gate structure in which photosense and selection functions are imparted to the photosensor.
FIG. 12A is a sectional view showing a structure of a double gate type photosensor 10, and FIG. 12B shows an equivalent circuit of the double gate type photosensor 10.
The double gate type photosensor 10 includes: a semiconductor thin film 11 of amorphous silicon; n+ silicon layers 17, 18 disposed on opposite ends of the semiconductor thin film 11; a source electrode 12 and drain electrode 13 respectively formed on the n+ silicon layers 17, 18; a top gate electrode 21 formed above the semiconductor thin film 11 via a block insulating film 14 and upper gate insulating film 15; a protective insulating film 20 disposed on the top gate electrode 21 and upper gate insulating film 15; and a bottom gate electrode 22 formed below the semiconductor thin film 11 via a lower gate insulating film 16, and is formed on a transparent insulating substrate 19 such as a glass substrate.
The double gate type photosensor 10 constituted in this manner includes: an upper MOS transistor constituted of the semiconductor thin film 11, source electrode 12, drain electrode 13, and top gate electrode 21; and a lower MOS transistor constituted of the semiconductor thin film 11, source electrode 12, drain electrode 13, and bottom gate electrode 22. As shown in the equivalent circuit of FIG. 12B, it can be considered that the photosensor is constituted by using the semiconductor thin film 11 as a common channel region and combining two MOS transistors each including a top gate terminal (TG), bottom gate terminal (BG), source terminal (S), and drain terminal (D).
The protective insulating film 20, top gate electrode 21, upper gate insulating film 15, block insulating film 14, and lower gate insulating film 16 are all constituted of materials which have a high transmittance against a visible light for energizing the semiconductor thin film 11. When a light incident upon the top gate electrode 21 is transmitted through the top gate electrode 21, upper gate insulating film 15, and block insulating film 14, and incident upon the semiconductor thin film 11, charges (positive holes) are generated and accumulated in a channel region.
FIG. 13 is a schematic constitution diagram of a photosensor system constituted by arranging a plurality of double gate type photosensors 10 in a matrix form.
As shown in FIG. 13, the photosensor system includes: a sensor array 100 in which a plurality of double gate type photosensors 10 are arranged in the form of a matrix of n rows×m columns; a top gate line 101 and bottom gate line 102 in which the top gate terminal TG and bottom gate terminal BG of each double gate type photosensor 10 are respectively connected in a row direction; a top gate driver 110 and bottom gate driver 120 connected to the top gate line 101 and bottom gate line 102, respectively; a data line 103 in which the drain terminal D of each double gate type photosensor 10 is connected in a column direction; and an output circuit unit 130 connected to the data line 103.
In FIG. 13, φtg and φbg denote control signals for generating a reset pulse φTi and read pulse φBi described later, and φpg denotes a precharge pulse which controls a timing to apply a precharge voltage Vpg.
In this constitution, as described later, when the top gate driver 110 applies a predetermined voltage to the top gate terminal TG, the photosense function is realized. When the bottom gate driver 120 applies the predetermined voltage to the bottom gate terminal BG, and an output voltage of the photosensor 10 is taken into the output circuit unit 130 via the data line 103 and outputted as serial data Vout, the read function is realized.
FIGS. 14A to 14D are timing charts showing a basic driving control method with respect to the photosensor 10 of one row in the photosensor system, and show a detection operation period (i-th row process cycle) in an i-th row of the sensor array 100.
First, a high-level pulse voltage (reset pulse; e.g., Vtg=+15 V) φTi shown in FIG. 14A is applied to the top gate line 101 of the i-th row, and a reset operation is performed to discharge charges accumulated in the double gate type photosensor 10 of the i-th row in a reset period Treset.
Subsequently, when a low-level (e.g., Vtg=−15 V) bias voltage φTi is applied to the top gate line 101, the reset period Treset ends, and a charge accumulation period Ta by a charge accumulation operation into the channel region is started. In the charge accumulation period Ta, the charges (positive holes) are accumulated in the channel region in accordance with the amount of light incident from a top gate electrode side.
In parallel with the charge accumulation period Ta, a precharge period Tprch elapses in which the data line 103 has a precharge voltage Vpg, a precharge pulse φpg shown in FIG. 14C is applied, and the drain electrode 13 holds the charges. Thereafter, when a high-level (e.g., Vbg=+10 V) bias voltage (read pulse φBi) shown in FIG. 14B is applied to the bottom gate line 102, the double gate type photosensor 10 is brought into an ON state and a read period Tread starts.
In the read period Tread, the charges accumulated in the channel region act in a direction in which the low-level voltage (e.g., Vtg=−15 V) applied to the top gate terminal TG having a reverse polarity is relaxed. Therefore, the voltage Vbg of the bottom gate terminal BG forms an n-channel, and a voltage VD of the data line 103 indicates a tendency to gradually drop with an elapse of time from the precharge voltage Vpg in accordance with a drain current. That is, the change tendency of the voltage VD of the data line 103 depends on the charge accumulation period Ta and the amount of received light. As shown in FIG. 14D, when the incident light is dark and has a small light amount, and a small amount of charges are accumulated, the voltage indicates a tendency to moderately drop. When the incident light is bright and has a large amount of light, and a large amount of charges are accumulated, a tendency to steeply drop is indicated. Therefore, the amount of irradiation light is converted based on a value of the voltage VD of the data line 103 after the elapse of a predetermined time.
In the photosensor system shown in FIG. 13, for example, rows of the sensor array 100 are driven/controlled in parallel with one another based on the above-described driving control method and at a timing at which times of the respective driving pulses do not overlap. Thereby, reset, precharge, and read operations are prevented from being executed at overlapped times. Moreover, the read operation can be performed before the reset operations in all the rows end, and a time required for the read operation of a two-dimensional image can be reduced.
In the above-described photosensor system, the double gate type photosensor is used as the photosensor. However, this is not limited. Even in the photosensor system in which a photodiode or phototransistor is used as the photosensor, similarly operation steps include “the reset operation→charge accumulation operation→precharge operation→read operation”, similar driving procedure is used, and the following similar problem occurs.
In the constitution of the above-described photosensor, the charges generated by the incident light are accumulated in the charge accumulation period, and bright/dark information of the subject image is detected based on the amount of charges. Therefore, when a subject is dark, and thus a small amount of charges are accumulated, the charge accumulation period is lengthened to obtain a sufficient detection sensitivity, and an image read sensitivity needs to be set to be high. On the other hand, when the subject is bright, and thus a large amount of charges are accumulated, the charge accumulation period is shortened in order to prevent the charges from being saturated, and the image read sensitivity needs to be set to be low. That is, it is necessary to appropriately set the image read sensitivity of the photosensor in accordance with the brightness of the subject so that the subject image is constantly satisfactorily read with appropriate sensitivity.
With various changes of place, time, and subject of the photosensor system for use, every time an ambient environment such as external illuminance, and/or a subject state changes, the brightness of the subject changes. Therefore, an appropriate image read sensitivity for satisfactorily reading the subject image changes every time. Then, it is necessary to perform an image read operation for obtaining and setting an appropriate image read sensitivity (hereinafter referred to as “read operation for sensitivity adjustment”) before a normal read operation of the subject image.
Examples of the read operation for sensitivity adjustment include: an operation of setting the image read sensitivities of the photosensors of the respective rows to be different from one another in the photosensor system including the photosensor array in which the photosensors are arranged in the matrix form; and reading a predetermined image for adjustment with a plurality of image read sensitivities by the image read operation for one screen corresponding to the photosensor array.
FIGS. 15A to 15J show one example of a timing chart of the driving control method in the read operation for sensitivity adjustment of the photosensor system.
As shown in FIGS. 15A to 15D, the driving control method first comprises: successively applying reset pulses φT1, φT2, . . . Tn−1, φTn from the first row of the top gate line 101 of the double gate type photosensor 10, for example, at a time interval Tdelay; and successively initializing the double gate type photosensors 10 of the respective rows. Subsequently, when the reset period Treset ends, the charge accumulation period Ta successively starts in each row, and the charges (positive holes) are accumulated in the channel region of the double gate type photosensor 10 in accordance with the amount of incident light.
Subsequently, as shown in FIGS. 15E to 15H, after the reset pulse φTn falls with respect to the last row (n-th row), from the n-th to first rows, for each row, the charge accumulation period Ta is changed at a time interval of the pulse interval Tdelay corresponding to a sum of the read period Tread, precharge period Tprch, and reset period Treset, and read pulses φBn, φBn−1, . . . φB2, φB1 are applied to the respective rows at a timing at which the time of the read period Tread of each row does not overlap.
Moreover, as shown in FIG. 15I, prior to the application of the read pulses φBn, φBn−1, . . . φB2, φB1 to each row, the precharge pulse φpg is applied in parallel within the charge accumulation period set for each row, and the precharge voltage Vpg is applied to the drain line 103 of the double gate type photosensor 10 of each row for the precharge period Tprch.
Thereby, following the precharge period Tprch, the read period Tread is started, and voltage changes VD1, VD2, VD3, . . . VDm corresponding to the charges accumulated in each double gate type photosensor 10 as shown in FIG. 15J are taken in and read out by a column switch 131 via the data line 103.
According to the driving control method, the images read with the same number of stages of different image read sensitivities as the number of rows can be acquired by the image read operation of one screen. In this case, since the charge accumulation period Ta of each row changes at a time interval twice Tdelay, a change range of the image read sensitivity can relatively be increased. It is possible to obtain an optimum value of detection sensitivity in accordance with a broad range of changes of the ambient environment or detection object.
However, in an actual use state, an optimum image read sensitivity does not very largely change depending on the state of use environment or subject in some case. In this case, even when the range for changing the image read sensitivity is narrowed to a certain degree in the read operation for the sensitivity adjustment, there is no problem as confirmed.
On the other hand, when the above-described driving control method is used in the read operation for sensitivity adjustment, and when the read operation for sensitivity adjustment is executed using all row ranges of an image read region of the photosensor array, the charge accumulation period, that is, the change range of the image read sensitivity becomes larger than the required change range of the image read sensitivity depending on setting conditions such as the time interval of the charge accumulation periods for each row and the number of rows of the photosensor array. In this case, there is a problem that useless control process is performed and time is wasted.
Moreover, to solve the problem, the execution timings of the reset and read operations are controlled so that the change range of the image read sensitivity is limited only to a necessary range. Then, there is a problem that the image read by the normal image read operation is deteriorated as described later in detail.